It is desirable in numerous industrial applications to armor the interior surfaces of metallic pipe with metallic cladding materials to protect against corrosion, abrasion, and/or surface contamination, and to provide improved impact resistance. For example, the processing of bitumen-laden sands (or “tar sands”) to produce synthetic crude oil typically involves mixing the tar sands with liquid to form a slurry, which is then piped to a processing plant. Because of its high content of sand and/or rock particles, the flowing slurry is extremely abrasive and will readily wear away the kinds of steels most commonly used for industrial piping. Metals such as stainless steel and chromium alloys have much greater resistance to abrasion (and corrosion) than common steels, but in most cases it would be prohibitively expensive to use piping made of such metals, particularly for larger diameter pipe.
A common and less expensive alternative in highly abrasive or corrosive industrial applications is to use ordinary steel pipe internally clad or armored with a more abrasion-resistant and/or corrosion-resistant material such as stainless steel, tungsten carbide, or a chromium alloy. The pipe is typically clad by depositing the cladding metal on the internal surfaces of the pipe using methods well known in the field of automatic and semi-automatic electric arc welding. Metal cladding wire is continuously fed from a wire spool to an applicator head (or “weld head”) disposed an optimal distance from the internal surface of a grounded pipe such that the introduction of an electrical current to the wire it will cause arcing between the wire and the pipe, in turn generating temperatures sufficient to melt the wire so that it will be deposited on and fused to the pipe. As with analogous welding procedures, the best results are typically achieved when this procedure is conducted in the “flat” position; i.e., with the surface to receive the molten metal being disposed beneath the weld head, as opposed to the “horizontal” and “overhead” positions (as those terms are commonly understood in the welding field).
The weld head is moved continuously relative to the pipe so that a continuous bead of metal cladding material is deposited on the pipe. This may be accomplished by moving the weld relative to the pipe, or vice versa. The mode of movement may be parallel to the pipe axis so to result in deposition of longitudinally-oriented cladding beads. It has been observed, however, that cladding beads oriented substantially parallel to the direction of flow appear to be more prone to abrasion than cladding beads oriented transversely to the flow direction. In addition, it has been observed that the application of cladding beads parallel to the flow direction tends to distort the cross-sectional shape of the pipe, due to residual stresses caused by differential cooling of the cladding beads.
For the foregoing reasons, it is preferable to use cladding beads that are of substantially circumferential orientation (i.e., transverse to flow direction), particularly in applications where the pipe is intended to carry highly abrasive materials such as tar sand slurries. It is possible to apply circumferentially-oriented cladding as a series of adjacent circular beads; however, this would involve repeated stopping and starting of the bead, which is inefficient and thus undesirable. As a practical matter, therefore, it is preferable to apply the cladding as a continuous helical bead.
Circumferential application of metallic cladding is relatively simple for straight sections of pipe. For example, the applicator head may be maintained in the “flat” position while the straight pipe is rotated around it. The pipe may be moved longitudinally relative to the applicator head after completion of each circumferential cladding bead, to allow the next bead to be deposited adjacent thereto. Preferably, however, the longitudinal movement of the pipe is continuous so that a continuous helical cladding bead will be deposited.
Helical application is considerably more difficult in the case of curved pipe sections such as pipe elbows. The procedure described above for cladding straight pipe is not workable with curved pipe. It is theoretically possible to apply helical cladding beads manually to a pipe elbow where the dimensions of the elbow permit manual access. However, this would entail an excessive amount of undesirable starting and stopping of the cladding bead due to the nearly constant need to reposition the elbow as the work progresses, particularly if it is being attempted to apply the cladding in the desirable “flat” position.
Because of the practical difficulties associated with helical cladding of elbows, longitudinal application methods are commonly used in spite of previously noted drawbacks. Moreover, this method is time-consuming, and therefore expensive. For these reasons, more efficient and economical means for helical cladding of elbows would be highly desirable.
Apparatus for helical application of cladding to curved pipe can be found in the prior art. Canadian Patent No. 2,282,134 and corresponding U.S. Pat. No. 6,234,383, issued to Harmat et al., disclose a rotatable framework having a curved cavity contoured to suit the shape of a curved pipe section which to receive internal metallic cladding. Guide tracks are provided along the sides of the cavity. Collars are fitted to each end of the pipe section, and the collars have wheels that engage the guide tracks so as to control the orientation of the pipe section as it moves into and through the cavity. One of the collars has a pair of guide pins that engage a guidance mechanism that is longitudinally movable so as to draw the pipe section progressively into the cavity as the framework is rotated about a longitudinal axis. The guidance mechanism has guide rails and other features adapted to compensate for the curvature of the pipe.
An elongate weld arm extends from one end of the framework into the cavity. At its outer end, the weld arm is fitted with a conventional weld head to which continuous welding wire is fed, for deposition on the interior surfaces of the pipe. The weld arm is geometrically configured such that it will not interfere with a pipe section passing through the cavity, and such that it moves in an eccentric path similar to a skipping rope as the framework is rotated about the longitudinal axis. During this rotation, the weld head remains in a fixed longitudinal position while at the same time describing an orbital path around the longitudinal axis. The weld head is connected to the weld arm in a fashion such that it remains in a fixed orientation (e.g., with the welding wire always feeding downward, in the “flat” position) regardless of the orbital rotation of the weld head.
To operate the apparatus, the guidance mechanism draws the pipe section into the cavity until the pipe reaches the weld head, with the weld head in position to engage the pipe's interior surface. The framework and weld arm are then cooperatively rotated, in coordination with the guidance mechanism which gradually draws the pipe further into the cavity. The circular rotation of the weld head, combined with coordinated longitudinal movement of the pipe through the cavity, results in a continuous helical bead of metal being deposited on the interior surface of the pipe.
Although the Harmat apparatus may be effective for helical deposition of internal cladding of curved pipe sections, it has certain drawbacks and disadvantages. Different guide tracks and other components of the apparatus must be used for different pipe sizes and curvatures. The Harmat apparatus is not readily suited for use with pipe sections having comparatively small diameters (e.g., 12-inch diameter or smaller) and/or comparatively small curvature radii, nor does it appear to be possible to use the apparatus to clad 90-degree elbows (or even 45-degree elbows). In addition, the Harmat apparatus cannot be used, without difficulty or at all, with a curved pipe section having a straight transition section.
For the foregoing reasons, there is a need for apparatus for helical deposition of metallic cladding to interior surfaces of curved pipe sections, where the apparatus is readily configurable for use with pipe sections of different diameters, without needing to change or replace any components of the apparatus. There is a further need for such apparatus which is readily adaptable for internally cladding curved pipe sections having smaller diameters and curvature radii than can be clad using known apparatus. In addition, there is a need for such apparatus which can internally clad not only the internal surfaces of curved pipe sections but also the internal surfaces of straight transition sections connected thereto. The present invention is directed to these needs.